The present invention relates to an apparatus and a method for producing uniform tension in a wide roll of waxed based ink ribbon during printing, to remove all wrinkles due to uneven surface tension across the ribbon. As ribbons increase in width they become more difficult to handle as a force must be applied across their width of varying magnitude to eliminate wrinkles. This is generally due to the nature of the polyester based carrier onto which the wax formula ink coating is uniformly applied. Unwinding of the ribbon for printing purposes onto a base paper requires this uneven force applied across the width, as the center has a tendency to sag and wrinkle under a uniform force applied across the width. Hence, a cambered circular or elliptical rod having a smooth surface is introduced acting against the ribbon to apply an uneven force, with an increment towards the center, counteracting the inherent sag in the ribbon as it is unwound. Wrinkled ribbons do not transfer ink onto the base paper at locations of these wrinkles, affecting the overall image quality.
Prior art in this field would involve thermal transfer printing mechanisms which use a waxed based ribbon to transfer an electronic image onto a base paper. A thermal print head is used as the heat source to melt the ribbon's ink coated on a carrier film, usually polyester, transferring ink to the base paper as selected heating elements comprising picture elements, or pixels, are individually addressed and heated to produce a tangible image on base paper.
The main problem associated with most prior art systems for this particular application is the narrow width of ribbon commonly found in most thermal transfer printing mechanisms. For printing widths of 8 1/2 inches or less, the thermal transfer ribbon handles suitably without the need for any ribbon tensioning devices. As printing widths increase to 24 inches and greater, there is considerable sag in the center of the ribbon which leads to printing problems as the ribbon wrinkles, also impairing document handling.
An example of a conventional printing mechanism related to the prior art, is shown in FIG. 1a to 1f of the accompanying drawings, according to Sakuragi et al. in U.S. Pat. No. 4,910,602.
In this printing mechanism according to FIG. 1a, a ribbon ink 1 unwounded from a core 2 is superposed on a recording paper 3 wound on a core 4. The ribbon ink 1 is directed between a thermal head 5 and a platen roller 6 via a tension roller 7 and is taken-up by a take-up shaft 8.
The prevention of ribbon 1 wrinkling conforming to Sakuragi's patent is accomplished by the tension roller 7. According to FIG. 1b, the roller 7 can rotate around its axis, having a bulged portion at its center, and gradually tapers lengthwise in both directions from the center.
In this case, as shown in FIG. 1c, the roller 7 induces in the ribbon ink 1 some tensile forces F1. The shape of the tensile forces F1 is determined by the gradual increase and decrease of the roller 7 diameter which is manufactured in such a way that the tensile forces F1 cannot be changed during rotation of the roller 7, thus lacking the ability to adapt the tensioning profile to various ribbon's elasticity. This is a major drawback of Sakuragi's patent.
Similarly, in another embodiment of Sakuragi's patent according to FIG. 1d, he uses two fixed side walls 9 and 10 in front of and behind the print head 5, which induce in the ribbon ink 1 some tensile forces F2 as shown in FIG. 1f. The variation of tensile forces F2 is determined by the profile of the side walls 9 and 10.
As shown in FIG. 1e, he uses a shaped edge profile peaking in the center for his walls. This solution does not conform exactly to the distribution of tensile forces in ribbon foils of a variety of compositions, as the device has no ability to adapt to various ribbons.
Further, the inclination of the sided walls 9 and 10 against ribbon's 1 surface is fixed; this also lacks the ability to adapt to the tensioning profile of various ribbon's elasticity. In his case, using a preset distribution of said forces will not necessarily work for different ribbon types.
In other words, Sakuragi's solution does not allow the flexibility of choosing different types of ribbon materials, limiting the user to type that works well with his device.
Hence, the primary function of the present invention is in the adapting to a variety of wide format ribbons to avoid any sag related complications affecting the image quality, allowing the flexibility of choosing different types of ribbon materials conforming to variations in elasticity.